1. Field of the Invention
The present invention relates to a thermoelectric device and a thermoelectric module that use thermoelectric material in the form of films to convert heat into electricity and vice versa.
2. Description of the Prior Art
A conventional thermoelectric device that is already in practical use is composed of p-type and n-type bulk thermoelectric materials arranged on a substrate, and these materials are connected alternately such that they are connected in series electrically. Such a bulk thermoelectric device is shaped like a flat plate, and has a low-temperature region formed on one of its top and bottom surfaces and a high-temperature region formed on the other.
By use of this bulk thermoelectric device, it is possible to cool an object by keeping a surface of the object in close contact with the low-temperature region of the device. Conversely, by producing a temperature difference between the top and bottom surfaces of the device, it is possible to obtain electric power. In this way, a thermoelectric device is used in thermoelectric cooling and heating for the purpose of cooling and heating various objects, as well as in thermoelectric power generation for the purpose of obtaining electricity from a heat source such as waste heat.
To make as efficient use of resources as possible and to achieve as great a cost reduction as possible, a modified type of thermoelectric device has been proposed that uses thermoelectric material in the form of films. For example, Japanese Laid-open Patent Application No. H8-32126 proposes a thermoelectric device composed of p-type and n-type semiconductor films that do not overlap one another and that are connected together by films of copper (Cu) laminated over them. On the other hand, Il-Ho Kim and Dong-Hi Lee propose, in Proc. 12th ICTEC, 1993, p. 328, a thermoelectric device that is composed of p-type semiconductor films, n-type semiconductor films, and Cu films and in which these films do not overlap one another. In this device, as shown in FIG. 9, the p-type semiconductor films 2 and the n-type semiconductor films 3, both made of thermoelectric material, are arranged on a substrate so as to form a radiating pattern, and these films are connected alternately by the Cu films 17 such that they are connected in series electrically.
This device has many junctions between the p-type semiconductor films 2, the n-type semiconductor films 3, and the Cu films 17. When the device receives a positive voltage and a negative voltage at its electrode films 18a and 18b, respectively, heat is absorbed at the junctions on the positive side of the p-type semiconductor films 2 (on the negative side of the n-type semiconductor films 3), whereas heat is released at the junctions on the negative side of the p-type semiconductor films 2 (on the positive side of the n-type semiconductor films 3). Accordingly, when the device is so energized that an electric current flows clockwise through the pattern shown in FIG. 9, heat is absorbed at the junctions between the p-type semiconductor films 2 and the n-type semiconductor films 3 (these junctions are placed near the center of the pattern), whereas heat is released at the junctions between the p-type semiconductor films 2 and the Cu films 17 and at the junctions between the n-type semiconductor films 3 and the Cu films 17 (these junctions are placed away from the center of the pattern).
A thermoelectric module is produced by covering the top surface of this thermoelectric device with electrical insulating material such as a high-polymer film or ceramic sheet. When such a thermoelectric module is energized, for example, in the manner as described above, the central part of the insulating material is cooled and a low-temperature region appears there, whereas the peripheral part of the insulating material is heated and a high-temperature region appears there. Thus, it is possible to cool an object, for example, by placing it in close contact with the low-temperature region of the insulating material. Conversely, by producing a temperature difference between the central and peripheral parts of the insulating material, it is possible to extract electricity between the electrode films 18a and 18b, that is, to generate electric power.
However, in the thermoelectric module composed of films of thermoelectric material as shown in FIG. 9, the heat-absorbing portion and the heat-releasing portion are both formed on a single plane. Therefore, it is not possible to construct, on this principle, a cascade-type thermoelectric module that is shaped like a flat plate and has a low-temperature region formed on one of its top and bottom surfaces and a high-temperature region formed on the other. Moreover, the thermoelectric module shown in FIG. 9, unlike the bulk thermoelectric device mentioned earlier, cannot cool or heat a wide-area surface uniformly, and thus its uses are limited.
Similarly, the thermoelectric device proposed in Japanese Laid-open Patent Application No. H8-32126 has many heat-absorbing and heat-releasing portions formed on a single plane, and, in addition, these heat-absorbing and heat-releasing portions are formed alternately along a straight line. As a result, this thermoelectric device cannot be used to selectively cool or heat an object, and thus its uses are limited mostly to power generation applications. Moreover, since the heat-absorbing and heat-releasing portions are placed close to one another, this thermoelectric device suffers from a heavy loss of heat through heat conduction and thus from poor thermoelectric conversion efficiency.